In a sound absorbing structure disclosed in US2005/0067220 corresponding to JP-2005-84693A, multiple Helmholtz resonators (i.e., resonant silencers) having different resonant frequencies are arranged to obtain a sound absorption characteristic over a wide frequency range.
Specifically, as shown in FIG. 6, the sound absorbing structure includes an inner member 101 and an outer member 102 that surrounds the inner member 101.
The inner member 101 defines an inner wall 104 having openings 103. The outer member 102 defines an outer wall 105 that extends along the inner wall 104 of the inner member 101.
The outer wall 105 forms bellows 106 to improve the sound absorption characteristic and flexibility of the sound absorbing structure. The bellows 106 and the inner wall 104 define cavities 107. The openings 103 of the inner wall 104 are open to the cavities 107.
A resonator, which is used in the conventional sound absorbing structure, is a device designed to absorb sound of a specific frequency. Therefore, to obtain sound absorption characteristics over a wide frequency range, a lot of resonators having different resonant frequencies are needed. As a result, the size of the sound absorbing structure is increased.
The present inventors have conducted an experiment to evaluate the effect of the conventional sound absorbing structure to reduce intake noise. In the experiment, a speaker was placed at one end of the inner member 101, and a microphone was placed at the other end of the inner member 101. Then, white noise was generated from the speaker, and sound caught by the microphone was analyzed by using a fast Fourier transform (FFT) analyzer. FIG. 7 shows a result of the experiment. As can be seen from FIG. 7, the sound absorption characteristic of the conventional sound absorbing structure has an excessive frequency region where the noise is excessively (i.e., unnecessarily) absorbed. Further, the sound absorption characteristic has a worse frequency region near the excessive frequency region. In the worse region, the noise is increased rather than reduced.